By including a variable resistor in series with the capacitor in the above circuit, frequency can be varied along with the duty ratio. As this resistor is involved in both charging and discharging periods, this does not affect the duty ratio.
Variable frequency PWM Using 555-Timer
This circuit can be used for speed control of motors, brightness control of lights etc., even if the specific frequency for proper operation is not known as in the case of dc fans or dc motors. It can be set by trial and error. The minimum time period of the waveform depends on the duty cycle potentiometer and maximum time period depends on the frequency potentiometer. Variable resistors should not be in their extreme positions or use fixed resistors in addition to a potentiometer to avoid this.